1. Field of the Invention
This invention relates to an image pickup lens suitable for mounting in a camera which uses a CCD or CMOS device as a pickup element.
2. Description of Related Art
One characteristic of this lens for mounting in a compact camera using a CCD or CMOS device as a pickup element is a short optical length. One lens of this type is for example the pickup lens disclosed in Japanese Patent Laid-open No. 10-206730.
However, in the pickup lens disclosed in this reference the distance from the aperture diaphragm plane to the second surface (the image-side surface) of the second lens is 5.3 mm, and the optical length is too long for use as a lens mounted in a compact camera using a CCD or CMOS device as a pickup element. In the pickup lens system disclosed in Japanese Patent Laid-open No. 10-206730, an aperture diaphragm is inserted between the first lens and the second lens. That is, the pickup lens system, disclosed in this reference employs a construction in which only one diaphragm is provided.
It is known that the position of an aperture diaphragm has important significance for lens design (see for example Fumio Kondo, Renzu no Sekkei Gihou, Kougaku Kougyou Gijutsu Kyoukai, 2nd edition Feb. 1, 1983). In other words, it is known that: (a) the entrance pupil position conjugate with the aperture diaphragm position is related to coma aberration, astigmatic aberration, distortion aberration, and similar, and is the basis for determining the third-order aberration coefficient; (b) when an aperture diaphragm is set at the position a distance t from the object-side surface (first surface) of the first lens counting from the object side (the first lens), measured along the optical axis moving toward the image side, if the value of B as defined by the equation (i) below is 0, then the basis is given for the Fraunhofer condition, according to which a sufficiently small aberration is realized;
B=Cxe2x88x92Stxe2x80x83xe2x80x83(i)
where C and S are constants related to the third-order aberration coefficient; and, (c) the basis is given for the Zinken-Sommer condition, according to which the closer the value of Z as defined in equation (ii) below is to 0, the better the aberration correction is guaranteed to be;
Z=St2xe2x88x922Ct+Axe2x80x83xe2x80x83(ii)
where C, S, and A are constants related to the third-order aberration coefficient.
In this way, when conducting a quantitative examination of aberration, the position of an aperture diaphragm plays an essential role, and is an important basic parameter of the lens system.
However, a short optical length is required of an image pickup lens for mounting in a compact camera as described above. In addition, an image pickup lens mounted in a compact camera as described above must be such that distortion of the formed image is not perceived visually, and such that various aberrations are corrected to small values as required by the integration density of the pickup element.
In the following explanation, xe2x80x9cvarious aberrations are corrected to amounts sufficiently small that distortion of the image is not recognized by visual perception, and sufficiently small as to satisfy the requirements of the integration density of the pickup elementxe2x80x9d is, for simplicity, represented by the phrase xe2x80x9cvarious aberrations are satisfactorily correctedxe2x80x9d or similar. An image for which various aberrations are satisfactorily corrected may be called a xe2x80x9csatisfactory imagexe2x80x9d.
An object of this invention is to provide an image pickup lens in which various aberrations are satisfactorily corrected, the optical length is short, and moreover sufficient back focus is maintained.
An image pickup lens of this invention which achieves the above object is configured by arranging, in order from the object side, an aperture diaphragm S1; a first lens L1; a second diaphragm S2; and a second lens L2. The first lens L1 has a meniscus shape with the concave surface facing the object side, and having positive refractive power. The second lens L2 has a meniscus shape with the concave surface facing the image side, and having negative refractive power.
Further, in the image pickup lens, at least one surface of the first lens L1 is aspherical, at least one surface of the second lens L2 is aspherical, and overall at least two lens surfaces are aspherical; and the following conditions are satisfied.
0.09 less than |f1/f2| less than 0.37xe2x80x83xe2x80x83(1)
1.33 less than |r1/f| less than 47.77xe2x80x83xe2x80x83(2)
3.08 less than |r1/r2| less than 113.12xe2x80x83xe2x80x83(3)
0.63 less than D/f less than 0.87xe2x80x83xe2x80x83(4)
Here f is the focal length of the entire system (the combined focal length of the lens system comprising the first and second lenses), f1 is the focal length of the first lens, f2 is the focal length of the second lens, D is the distance from the aperture diaphragm plane to the second surface (image-side surface) of the second lens (lens center length), r1 is the radius of curvature of the object-side surface of the first lens L1 in the vicinity of the optical axis (axial radius of curvature), and r2 is the radius of curvature of the image-side surface of the first lens L1 in the vicinity of the optical axis (axial radius of curvature).
The aperture diaphragm S1 of this invention is positioned between the object and the first lens L1. In other words, the aperture diaphragm S1 is set on the outside of the first lens L1, that is, in front of the first surface (the object-side surface) of the first lens. This aperture diaphragm S1 forms an incidence plane. A second diaphragm S2 provided between the first lens L1 and the second lens L2 is inserted in order to cut out so-called flare, which is light which strikes the peripheral edge of a lens or similar and is irregularly reflected.
Next, the significance of the above condition equations (1) through (4) is explained.
The above condition equation (1) determines the power distribution of the first lens L1 and second lens L2; if |f1/f2| falls below the lower limit, the power of the first lens L1 is stronger and the power of the second lens is weaker, so that correction of the spherical aberration, coma aberration, and distortion aberration produced by the first lens becomes difficult. And if |f1/f2| exceeds the upper limit, the power of the first lens L1 becomes weaker, and consequently the power of the second lens must be increased in order to shorten the combined focal length f and back focus bf(distance from the point of intersection of the image-side surface of the second surface of the second lens with the optical axis, to the point of intersection of the light-receiving surface with the optical axis) of the lens system. Hence correction of the distortion aberration and coma aberration produced by the second lens L2 becomes difficult. As a result, if |f1/f2| falls below the lower limit or rises above the upper limit, a satisfactory image cannot be obtained. Consequently using an image pickup lens of this invention which satisfies the condition equation (1), a satisfactory image can be obtained.
The above condition equation (2) sets the range for the value of |r1/f| when the radius of curvature r1 on the object side of the first lens L1 is normalized by the combined focal length f for the pickup lens system. If |r1/f| falls below the lower limit, coma aberration increases, and if an attempt is made to correct this, distortion aberration results. Hence the need arises for means to cut rays which pass through the peripheral portions of lenses, and as a result the image is darker.
On the other hand, if |r1/f| exceeds the upper limit, astigmatic aberration and coma aberration are increased, and moreover the lens thickness is increased, so that a satisfactory image cannot be obtained over a broad angle range. That is, if the radius of curvature r1 on the object side of the first lens L1 is set so as to satisfy condition equation (2), it becomes easy to correct the coma aberration, astigmatic aberration and distortion aberration of the pickup lens, the pickup lens can be made more compact while, maintaining broad angles, and in addition image brightness can be preserved.
The above condition equation (3) stipulates the ratio of the radii of curvature r1 and r2 of the first lens L1; if |r1/r2| falls below the lower limit, the optical length increases, or the lens diameter increases, or distortion aberration is increased. On the other hand, if |r1/r2| rises above the upper limit, coma aberration increases. In other words, if calculations are performed so as to satisfy condition equation (3), correction of the coma aberration and distortion aberration of the pickup lens becomes easy, and moreover the pick lens can be made more compact.
The above condition equation (4) stipulates the range for the value obtained by normalizing the distance D from the point of intersection of the aperture diaphragm S1 with the optical axis to the point of intersection of the second surface (image-side surface) of the second lens with the optical axis by the combined focal length f for the lens system. When this lens system is actually used, cover glass or similar is inserted behind (on the image side of) the second lens L2. The value of D/f provides an indicator of the magnitude of the optical length (the distance from the incidence aperture position to the imaging plane) of the entire image pickup lens of this invention, with the cover glass or other optical component added. By keeping this value within the range of the condition equation (4), the optical length as calculated assuming the use of cover glass or similar can be kept to a length within the range allowable for practical application.
As is clear from the first embodiment through the fourth embodiment described below, the four conditions stipulated by the condition equations (1) through (4) make possible the realization of an image pickup lens in which various aberrations are satisfactorily corrected, which has an optical length of 6 mm or less (D value of 2.98 mm or less), and which affords excellent productivity.
Looking again at the pickup lens system disclosed in Japanese Patent Laid-open No. 10-206730, as has already been explained, an aperture diaphragm is set between the first lens and the second lens. On the other hand, in this invention the aperture diaphragm position is in front of the first lens. As a result, the manner in which various aberrations appear is clearly different for this invention and for the pickup lens system disclosed in Japanese Patent Laid-open No. 10-206730, and the lens system disclosed in the above reference can be understood to be structurally different from the lens system of this invention.
Further, it is preferable that in an image pickup lens of this invention, all component lenses be formed from plastic material (a polymer material which can be molded and shaped by plastic deformation under the application of heat or pressure, or both, and which is transparent to visible light).